Wood preserving composition



2,816,057 PatentedDec. 10, 1957 Fire Woon PRESERVING coMPosrrioN CharlesDonald Cook, Castleford, England, assiguor to Hicksons Timberimpregnation Co. (G. B.) Limited, Castleford, Engiand, a British companyNo Drawing. Application July 15, 1955, Serial No. 522,366

Claims priority, application Great Britain February 3, 1955 6 Claims.(Cl. 16738.5)

This invention is concerned with improvements in or relating to woodpreservative compositions in powder form and is more particularlyconcerned with such wood preservatives as contain arsenic compounds.

The use of arsenic compounds in wood preservative compositions is wellknown and in particular the use of compositions containing arsenicpentoxide, an alkali metal dichromate and copper and/or zinc salts hasbeen proposed.

The use of arsenic pentoxide in free form, has however been known togive rise to various difliculties, thus, for example, arsenic pentoxideis diflicult to handle by virtue of its tendency to deliquesce, its useis attended by health hazards, and the material cannot be ground readilyto a fine powder.

Initially dry compositions containing arsenic pento-xide, in free form,and one or more ingredients containing loosely combined water, forexample copper sulphate penta-hydrate, in general quickly liquefy orbecome pasty even when packed in air-tight containers. Thus such acomposition when packed in drums or other containers, becomes ingeneral, virtually impossible to handle satisfactorily even after ashort storage period. Thus not only are the handling difiicultiesincreased but the health hazards are likewise increased. The reason forthis change in state has been traced to the interchange of water betweencompounds with loosely combined water (as water of crystallisation), forexample copper sulphate penta-hydrate, and the very deliquescent arsenicpentoxide. The rate of interchange of the water is promoted byincreasing the temperature on storage, for example the rate ofinterchange will be much greater in tropical .conditions than in moretemperate climates.

By compound with loosely combined water I mean compounds such as, forexample, copper sulphate pentahydrate, which can lose a proportion oftheir water of crystallisation at temperatures of, for example, 20 C.and in particular at temperatures of 3540 C., that is temperatures whichcan be experienced in tropical or subtropical areas, in which thepreservative might either be stored or through which it might betransported. In the presence of a deliquescent compound such as sodiumdichromate or arsenic pentoxide the loss of water from, for example,copper sulphate penta-hydrate is accelerated with the result that themixture will agglomerate or become a paste.

As a result the use of compositions containing arsenic pentoxide, infree form, copper salts and chromates and/ or dichromates has notdeveloped to any great extent, although there are clear indications thatsuch prepa rations would be advantageous from a preservative point ofview.

Previously the above mentioned disadvantages have been reduced to someextent by obtaining the several ingredients separately and making up thepreservative at the time of use. Such a procedure is, however,inconvenient and may, inter alia, give rise to incorrect mixing of theingredients and hence poor treatments. A further disadvantage is thatthe plant operator is exposed to the health hazards associated with theuse of arsenic pentoxide either in solid or liquid form. Again thesedisadvantages have also been in part reduced by manufacturing aconcentrated solution. Such a procedure oflers certain problems onstorage and the concentrated solution is not as easy to handle as thefree flowing powder at the treatment plant. Moreover the handling of aconcentrated liquid containing arsenic pentoxide has been found tosubject the plant operator to health hazards.

It is an object of the present invention to provide a wood preservativecontaining arsenic in the form of a free flowing powder wherein theabove-mentioned disadvantages are reduced and which, for a reasonabletime, remains in a form, which can be handled more readily, particularlyat the treatment plants, than hitherto known compositions containingarsenic.

My investigations have shown that alkali metal arsenates of a certaintype hereinafter referred to as low pH alkali arsenate free flowingpowder may be conveniently used in the formulation of a woodpreservative in powder form with improved results from theabove-mentioned point of view.

I use the term low pH alkali arsenate free flowing powder to designatean alkali metal or ammonium arsenate, preferably a sodium arsenate whichin 1.2 w./v. aqueous solution gives rise to a pH of less than 4.6,preferably less than 3.6, but not less than 2.0. The symbol w./v.indicates weight/volume on the metric scale.

Various grades of low pH alkaii arsenate free flowing powder can beprepared and I prefer to employ a low pH sodium arsenate free flowingpowder containing from 60-70% by weight of combined arsenic calculatedas arsenic pentoxide (AS205) based on an oven dry weight at C. It is tobe noted that it may be desirable to add to the low pH alkali arsenatefree flowing powder, during its manufacture, or at a later stage,various diluents in order to regulate the characteristics, such as thepH and consistency, of the final product. Thus it has been found to beadvantageous to add sodium sulphate to the low pH sodium arsenate freeflowing powder to facilitate grinding. The incorporation of sodiumbisulphate has also been found to be advantageous in facilitating theproduction of a free flowing powder and for regulating the pH of thepreservative composition.

Such low pH arsenates can be preparedas free flowing powders and have alesser tendency to pick up moisture than arsenic pentoxide.

Accordingly the invention comprises a wood preservative composition in apowdered free flowing form adapted,

to be dissolved in water prior to use for wood preservation comprisingin admixture a low pH alkali arsenate free flowing powder as abovedefined together with a non-delinquescent copper and/ or Zinc salt andone or more of the following substances, namely potassium dichromate,ammonium dichromate and anhydrous sodium dichromate.

It is preferred that the wood preserving composition according to theinvention should not contain any deliquescent compounds.

The term deliquescent as used herein is used to designate such compoundsfor example as arsenic pentoxide or sodium dichromate which may absorbmoisture from the surrounding atmosphere to such an extent that.

they become liquified.

A wood preservative composition according to the present invention mayalso contain other compounds which have a preserving action. Suchcompounds are,

By restricting the pH of the compositions according to the invention tonot less than 2.0 or 2.1 it has been discovered that in addition to theabove advantages of reducing the deliquescence and producing a freeflowing powder, the rate of sludging of the preservative when in contactwith the wood is also reduced. The compositions according to theinvention are therefore preferably such as have in aqueous 4% solution apH within the range of from 2.0-2.8, preferably 2.1-2.25.

Alkali metal dichromates which can be used in the manufacture of woodpreservative compositions include the dichromates of potassium andsodium. Of these we prefer to use potassium dichromate. When sodiumdichromate is used there are certain disadvantages. Thus a mixturecomprising a low pH alkali arsenate free flowing powder, sodiumdichromate and copper sulphate penta-hydrate can in general be storedfor a reasonable length of time in a temperate climate when packed inair-tight containers. However, once the product is exposed to theatmosphere it quickly agglomerates or liquifies and becomes difficultand hazardous to handle. If, however, the anhydrous sodium dichromate isreplaced by potassium dichromate the mixture can be exposed to theatmosphere for a substantially longer period without seriousagglomeration and pasting.

Where copper salts are to be used copper sulphate penta-hydrate may beused providing a low pH alkali arsenate free flowing powder containingless than 70% by weight of combined arsenic, calculated as arsenicpentoxide (AS205) based on an oven dry weight at 100 C., is employed.

In order that the invention may be well understood the followingexamples are given by way of illustration only:

Example 1 A wood preserving composition was made up as follows:

Percent by weight CuSO H O 28 NaHSO 15 NaH AsO (pH 4.5) 20 Kzcl'zoq Thiscomposition remained as a reasonably free flowing powder when stored inair-tight containers for considerable periods and could be exposed toatmospheric moisture pick up for a reasonable period without seriouscaking or congealing.

Example 2 A wood preserving composition was made up as follows:

Low pH sodium arsenate free flowing powder (pH 2.2-As O 2H O67%)containing sodium bisulphate 27 Cl-ISO45H3O This mixture can be storedfor prolonged periods in temperate regions without substantial caking.

Such a mixture is stable to storage for several weeks in a temperateclimate and could be exposed to atmospheric moisture pick up for areasonable period without serious caking or congealing.

Example 5 A composition was made up as follows:

Percent by weight 41 K Cr 0 CuSO 5H O Low pH sodium arsenate freeflowing powder (pH 2.2) containing sodium bisulphate: As O 2H O content70% 26 Such a mix is stable to storage for several weeks in a temperateclimate and could be exposed to atmospheric moisture pick up for areasonable period without serious caking or agglomeration.

Example 6 A composition was made up as follows:

Percent by weight CuSO 5H O 35 Low pH sodium arsenate free flowingpowder (pH 2.2As O 2H O67%) containing sodium bisulphate 26 Anhydroussodium dichromate 39 Such a composition remains stable for a longerperiod than a similar composition containing arsenic pentoxide in placeof the low pH sodium arsenate free flowing powder.

Example 7 I A composition was made up as follows:

Percent by weight (NH4)2C12O7 Low pH sodium arsenate free flowing powder(pH 2.2-As O 2H O67%) containing sodium bisulphate 26 Such a mixture isstable to storage for several weeks.

7 I claim:

1. A wood preservative composition in a powdered free flowing formadapted to be dissolved in water prior to use for wood preservationcomprising in admixture a member of the group consisting of alkali metaland ammonium arsenates which in 1.2% w./v. aqueous solution give a pH ofless than 4.6, a member of the group consisting of non-deliquescentcopper and zinc salts, and a member of the group consisting of potassiumdichromate, ammonium dichromate and anhydrous sodium dichromate.

2. A wood preservative composition as claimed in claim 1 which has in 4%W./v. aqueous solution a pH within the range of from 2.0-2.8.

3. A wood preservative composition as claimed in claim 2 which containssodium bisulphate.

4. A wood preservative composition as claimed in claim 3 having in 4%w./v. aqueous solution a pH within the range of from 2.1-2.25.

5. A wood preservative composition as claimed in claim 4 which containssodium bisulphate.

6. A wood preservative composition in a powdered free flowing formadapted to be dissolved in water prior to 5 6 use for wood preservationcomprising in admixture a References Cited in the file of this patentmember of the group consisting of alkali metal and am- UNITED STATESPATENTS monium arsenates which in 1.2 W./v. aqueous solution give a pHwithin the range of 2.0-3.6, a member of the 1520452 Ciumn 1927 groupconsisting of copper sulphate penta-hydrate and zinc 5 1365948 butt July1932 sulphate hepta-hydrate, and a member of the group con- FOREIGNPATENTS sisting of potassium dichromate, ammonium dichromate 993,654France Nov 5, 1951 and anhydrous sodium dichromate.

1. A WOOD PRESERVATIVE COMPOSITION IN A POWDERED FREE FLOWING FORMADAPTED TO BE DISSOLVED IN WATER PRIOR TO USE FOR WOOD PRESERVATIONCOMPRISING IN ADMIXTURE A MEMBER OF THE GROUP CONSISTING OF ALKALI METALAND AMMONIUM ARSENATES WHICH IN 1.2% W./V. AQUEOUS SOLUTION GIVE A PH OFLESS THAN 4.6 A MEMBER OF THE GROUP CON-NSISTING OF NON-DELIQUESCENTCOPPER AND ZINC SALTS, AND A MEMBER OF THE GROUP CONSISTING OF POTASSIUMDICHROMATE, AMMONIUM DICHROMATE AND ANHYDROUS SOLDIUM DICHROMATE.